This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 98 05299 filed in France on Apr. 28, 1998; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus for treating the internal surface of a gas bottle. It furthermore relates to a gas bottle whose internal surface has been treated by the method.
2. Description of the Related Art
Bottles intended for storing gases are made of a material, generally a metallic material, which is compatible with the properties of the gas stored.
Current specifications with regard to the in-bottle storage of high-purity gases require very low levels of gaseous or metallic impurities in the bottles. These levels may be as low as several parts per billion, or even parts per trillion, depending on the nature of the gas.
In order to ensure impurity levels as low as possible, it is known to carry out a treatment of the internal surface of the bottle, especially for the purpose of reducing interactions between the gas and the surface. These interactions are in fact sources of contamination of the gas and of degradation of the bottle.
Several techniques for preparing bottles have been used until now. Known techniques include, for example:
electroneutralization or chemical cleaning, eliminating the active sites on the internal surface, it being possible for this cleaning to be carried out in an ultrasonic bath;
mechanical polishing (microshot peening, lapping, sandblasting, etc.) and electropolishing, which eliminates the tearing marks and defects;
chemical deposition or vapour deposition, covering the internal surface of the bottle with a protective layer which is more chemically tailored to the gas; and
passivation, allowing the wall to be rendered chemically inert.
These preparation techniques are effective at various levels, especially with regard to improving the roughness or the cleanliness, to removing impurities, and to reducing the level of outgassing.
It is possible to achieve high-quality surface finishes using conventional treatment methods. However, to do this it is necessary to increase the number of treatment operations and to combine several methods in order to compensate for the drawbacks resulting from each of them. This results in a high manufacturing cost per bottle and involves lengthy treatment times.
A mechanical polishing technique consists, for example, in microshot peening the internal surface of the bottle.
For this purpose, the bottle is placed vertically, with the mouth pointing downwards. A tube for injecting glass balls and for spraying them is introduced into the bottle along its axis. Since the bottle is rotated about its axis, the glass balls are thrown against the internal surface of the bottle from the end of the tube. The tube is moved axially along the length of the bottle so as to treat the bottle over its entire length.
This polishing technique, as in the other polishing techniques, has the drawback of creating microcavities in the surface of the compressed metal, these being likely to trap impurities which may contaminate the gas contained in the bottle.
Bottle treatments using chemical cleaning entail, in succession, washing operations in acid baths, and then in base baths followed, at each step, by rinsing operations using deionized water, and, finally, a bottle-drying operation. The treatment times may thus amount to several hours per bottle, and consume large quantities of products. These treatments require expensive plants, especially in order to recycle the rinsing water.
Ultrasonic chemical cleaning consists of a succession of immersions of the bottles in baths of various types, in the presence of ultrasound.
The first phase comprises immersing the bottles in a leaching bath based on phosphoric acid at a temperature of 50xc2x0 C. to 70xc2x0 C. in the presence of ultrasound.
In a second phase, the bottles are rinsed before being dried in a stream of filtered nitrogen maintained at approximately 60xc2x0 C.
The rinsing phase includes a first step of two successive immersions in two tanks filled with water.
The bottles are then exposed to trichlorotrifluoroethane.
Document EP-A-0,753,380 describes a method for treating a pressurized-gas container which entails a succession of steps of the type of those mentioned above.
Likewise, document FR-A-1,603,506 describes a method for mechanically shaping the internal surface of hollow components.
Finally, EP-B-0,380,387 describes an apparatus for cleaning a surface using a laser beam. However, this apparatus is only used for surfaces that are easily accessible, because of the use of a hand-held component for pointing the laser beam. Thus, the apparatus cannot be used to treat the inside of a bottle.
The methods described above all have the drawback of introducing new elements on the internal surface of the bottle (for example: silica deposit during microshot peening, traces of acids and of bases) which correspondingly constitute additional impurities. The treatments normally employed by conditioning centres combine a phase of microshot peening with a subsequent treatment phase using perchloroethylene, so as to remove the greases (hydrocarbons) and the deposits left by the microshot peening. Because of the new regulations with regard to solvents, this substance will shortly no longer be able to be used.
The object of the invention is to provide a method and an apparatus for treating the internal surface of a gas bottle, which is easy and quick to implement, while guaranteeing satisfactory treatment of the internal surface of the bottle.
For this purpose, the subject of the invention is a method for treating the internal surface of a gas bottle, characterized in that it includes the following steps:
a) an incident laser treatment beam is introduced into a bottle through its mouth, approximately along the axis of the bottle;
b) the laser beam is deflected in the bottle onto the internal surface of the bottle;
c) a relative rotation between the bottle and the deflected laser beam is made approximately about the axis of the bottle; and
d) a relative displacement between the bottle and the deflected laser beam is made so as to scan most of the internal surface of the bottle with the deflected laser beam.
According to particular modes of implementation, the method includes one or more of the following characteristics:
at step d) of relative displacement between the bottle and the laser beam, two successive scans of most of the internal surface of the bottle are made, the first scan by the laser beam producing an ablation of the surface layer of the internal surface of the bottle, under the action of an athermal shock wave, followed by a second scan by the laser beam producing a thermal effect at the surface of the bottle, resulting in surface remelting of the latter;
the relative displacement comprises a translational relative movement of the deflected laser beam with respect to the bottle approximately along the axis of the bottle;
the relative displacement comprises modifying the angle of deflection of the deflected laser beam with respect to the axis of the bottle;
a cleaning gas is injected into the bottle during the scanning of its internal surface by the deflected laser beam;
the cleaning gas injected into the bottle is continuously sucked out; and
an amalgam of protective metals is sprayed onto the point of impact of the deflected laser beam on the bottle.
The subject of the invention is also a gas bottle, characterized in that it has an internal surface treated by the method described above.
The subject of the invention is also an apparatus for treating the internal surface of a gas bottle, characterized in that it includes:
a) means for introducing an incident laser beam inside a bottle through its mouth, approximately along the axis of the bottle;
b) means for deflecting the laser beam in the bottle onto the internal surface of the bottle;
c) means for generating a relative rotation between the bottle and the deflected laser beam approximately about the axis of the bottle; and
d) means of relative displacement between the bottle and the deflected laser beam so as to scan most of the internal surface of the bottle with the deflected laser beam.
Depending on particular embodiments, the apparatus includes one or more of the following characteristics:
it includes means for making, during the relative displacement between the bottle and the laser beam, two successive scans of most of the internal surface of the bottle, a first scan by the laser beam producing an ablation of the surface layer of the internal surface of the bottle, under the action of an athermal shock wave, followed by a second scan by the laser beam producing a thermal effect at the surface of the bottle, resulting in surface remelting of the latter;
the said means of relative displacement comprise means of translational relative movement of the deflected laser beam with respect to the bottle approximately along the axis of the bottle;
the said means of relative displacement comprise means for modifying the angle of deflection of the deflected laser beam with respect to the axis of the bottle;
the means for modifying the angle of deflection of the laser beam comprise a prism pivoted about an axis approximately perpendicular to the axis of the bottle, which prism is placed so as to receive the incident beam via an entrance face and to send on the deflected beam via an exit face;
the prism is a triangular-base prism and its third face, complementary to the entrance and exit faces for the laser beam, has a coating with a high reflection coefficient;
the prism is a triangular-base prism and its third face, complementary to the entrance and exit faces for the laser beam, is coupled to a mirror, the reflecting face of which is directed towards the inside of the prism;
the prism is a triangular-base prism and its third face, complementary to the entrance and exit faces for the laser beam, is coupled to a mirror, the reflecting face of which is directed towards the outside of the prism;
it includes means for injecting a cleaning gas into the bottle during the scanning of its internal surface by the deflected laser beam;
it includes means for continuously sucking out the cleaning gas injected into the bottle; and
it includes means for spraying an amalgam of protective metals onto the point of impact of the deflected laser beam on the bottle.